The present invention relates to self-contained breathing apparatus such as may be used for underwater diving or in other hostile environments in which a user may need a supply of breathable gas. Such uses include fire fighting where the atmosphere may be heavily polluted with combustion products and noxious gases, other industrial environments where the atmosphere may be polluted or otherwise unbreathable, or at high altitude where the atmosphere itself is too thin or effectively non-existent.
Although applicable to a wide range of other uses the present invention will be described hereinafter with particular reference to its application to underwater breathing apparatus for diving applications. It will be understood, however, that this description is provided without prejudice to the generality of the invention or its range of applications.
It is well known to provide divers with self-contained underwater breathing apparatus in order to prolong the time for which they can remain below the surface of the water. The most widely used self-contained breathing apparatus comprises a rigid container within which is housed a supply of compressed air which is allowed out of the container via a high pressure or first stage regulator and directed through a flexible hose to a mouthpiece containing a demand valve including a second stage regulator which acts automatically to open and close as the diver inhales and exhales. Such systems are known as open-circuit breathing apparatus because exhaled gas is allowed to pass directly out into the marine environment so that a stream of bubbles is emitted upon each exhalation. If the compressed gas breathed from the gas container is air a large proportion of the exhaled gas will constitute nitrogen which is present in air in an approximate ratio of 4:1 with oxygen as is well known. In other words 80% of the air which is breathed by the diver, and therefore 80% of the content of the compressed air container, or air bottle, comprises little more than a vehicle for the oxygen some of which is converted to carbon dioxide during its residence in the lung. Thus 80% of the breathed gas is not really needed by the body except to dilute the oxygen. It is not possible to breath pure oxygen below 10 m since at higher pressures oxygen is toxic.
Proposals have in the past been made for so-called closed circuit or xe2x80x9cre-breatherxe2x80x9d apparatus in which the carbon dioxide content of exhaled air is removed from the exhaled air outside the body, fresh oxygen is introduced to replace that consumed, and the thus-reconditioned air returns to the diver for re-breathing. In this way it is necessary for the diver only to carry two or three lungfuls of nitrogen sufficient to circulate around the closed circuit. Such a system is described, for example, in U.S. Pat. No. 4,964,404 to William C Stone and in U.S. Pat. No. 3,555,098 to John W Kanwisher and Walter A Starck II. These Patentees were not the first to devise closed circuit re-breather apparatus, however, it being known that so-called xe2x80x9cfrogmenxe2x80x9d used re-breather apparatus during World War 2 in order to avoid the tell-tale bubbles rising to the surface upon exhalation in an open-circuit system such as the traditional compressed air bottle arrangement described above. U.S. Pat. No. 4,964,404 describes an improved such mixed gas breathing apparatus in which a container for exhaled gas (the so-called counterlung) is formed in two parts, a first part communicating with a hose leading from a mouthpiece to a carbon dioxide removal filter, and a second part in the line between the carbon dioxide removal filter and the mouthpiece. The carbon dioxide removal filter in the system described in U.S. Pat. No. 3,556,098 includes a chamber housing oxygen partial pressure sensors used to detect the oxygen content in the exhaled gas and to reinstate the oxygen balance by introducing oxygen through a valve controlled indirectly by the sensors. The oxygen sensor system is described as comprising three sensors with the average value of the sensor signals being taken to produce the control signal. Three sensors are used on the grounds that the appropriate introduction of the right amount of oxygen is so critical, in these circumstances, that it is not possible safely to rely on the signal from a single sensor or even two sensors because any failure of a sensor may not be detected or recognised sufficiently quickly to prevent inadequate oxygenation of the circulating gas, or excess oxygenation depending on the nature of the failure. The argument presented for utilising three sensors is that by taking the average of three sensors the departure from the correct value introduced by a single faulty sensor is minimised. The effect of a faulty sensor on the average value is limited by electronically xe2x80x9cclippingxe2x80x9d the values to predetermined maximum and minimum values. The three sensors are monitored so that should one start to produce a signal which differs materially from that produced by the other two an alarm is indicated and the dive can be aborted. This strategy is based on the fact that the probability of two sensors being faulty is low, and the probability of two sensors being faulty at the same time is lower and can be reduced even further by taking remedial action immediately a faulty sensor is detected.
However, although this makes concessions to absolute safety by using an alarm signal upon departure of one sensor beyond a predetermined threshold from the other two, this results in the need for the diver to make a judgement on whether the other two sensors are performing properly and risks disruption to the diving activity unless the remaining two sensors are so clearly providing the correct control signal that the diver can come to the conclusion that he can safely ignore the third. The safety strategy adopted by W Stone in U.S. Pat. No. 4,964,404 is further reinforced by the provision of two entirely separate closed circuit re-breather systems each having front and back counterlungs and each being adapted to utilise components of the other system in the event of failure. Such 100% redundancy is necessitated by the chosen strategy in the management of the sensor signals and results in considerable additional equipment expense and bulk.
The present invention seeks to provide self-contained breathing apparatus of the closed-circuit re-breather type in which an improved strategy for management of the oxygen sensors is adopted which, whilst recognising the possibility of failure of an oxygen sensor, monitors the operation in a more practical manner and avoids the necessity for the duplication of all the components without loss of safety. Indeed, safety of the diver remains of paramount importance and numerous features of the apparatus formed in accordance with the present invention are directed at minimising the risk to the diver whilst nevertheless avoiding the need unnecessarily to resort to open circuit emergency breathing due to minor malfunctioning of equipment.
According to a first aspect of the present invention, there is provided self-contained breathing apparatus of the type having a container for receiving exhaled gas, means for removing carbon dioxide from the exhaled gas, sensor means for detecting the oxygen content of the exhaled gas and means for injecting oxygen into the exhaled gas to reinstate the oxygen content so as to lie within a desired range for re-breathing, in which the signals from the oxygen sensor means are delivered to two independent signal processing circuits which are interconnected in a primary and secondary relationship with the primary signal processing circuit acting in use to control the operation of a solenoid valve for injection of oxygen into the exhaled gas and the secondary signal processing circuit acting in use to display information concerning the sensor output signals to provide confirmation of the satisfactory operation of the master signal processing circuit.
Preferably the said signal processing circuits are interconnected with a signal line and the secondary signal processing circuit is able constantly to monitor the operation of the primary signal processing circuit.
Through this interconnecting signal line the two signal processing circuits are able to communicate with one another, the processors incorporated therein being programmed to check, upon being switched on, whether any signals are being received from the other circuit. If not it acts as the primary controller and commences transmitting signals to the other signal processing circuit to identify this condition. Each signal processing circuit has an independent on/off switch and is programmed to adopt the role of primary circuit if the other signal processing circuit is not switched on or is malfunctioning for example due to power failure at the primary circuit or if the diver should (perhaps inadvertently) switch off the original master.
The present invention also comprehends self-contained breathing apparatus of the type in which exhaled gas is reconditioned by the introduction of oxygen to prepare it for re-inhalation and the oxygen content is constantly monitored by oxygen sensors the output from which is used to control the reintroduction of oxygen into the gas to prepare it for re-breathing, in which the oxygen sensor means comprise three independent sensors housed in a chamber through which the gas to be reconditioned flows in use of the apparatus, and the outputs of the sensors are delivered to signal processing circuits which act to determine the value of the partial pressure of oxygen in the gas in the chamber by taking the average value of whichever of the two sensor outputs are nearest to one another in value.
The argument for using three sensors is developed in U.S. Pat. No. 3,556,098 although a rather different operational strategy is utilised in the present invention. If only one sensor were present there would be no way of determining whether its output is right or wrong, and even if two sensors are used there is still no way in which a utiliser can determine, if the two sensors provide different outputs, which one is correct and which one is not. In U.S. Pat. No. 3,556,098 three sensors are utilised and the average of all three taker, with the output signals being limited or xe2x80x9cclippedxe2x80x9d so that should they depart from a predetermined set value by more than this predetermined deviation the effect which a sensor producing an erroneous signal will have on the average is minimised. This nevertheless allows the erroneous sensor to have some influence on the average and, furthermore, introduces a potentially dangerous situation if all sensors correctly indicate a value outside the preset range since the system will act to limit their signals to the preset limits. In the system according to the present invention the signal from the sensor which differs from each of the other two by the greatest amount is ignored; this allows a diver still to make use of the rebreather equipment when a faulty sensor is detected (this could be an alarm signal) whilst still having reasonable confidence that the equipment as a whole is functioning correctly so that should it be necessary to undertake decompression stops or if there is an extended route (for example out of an underwater cave system) before the diver can surface it is not necessary to resort to the open circuit breathing equipment which may not have sufficient capacity for an extended exit procedure.
Preferably the oxygen control system acts to maintain a nominal oxygen partial pressure at a lower range in the region of 0.7 bar. In practice the equipment may be set to a lower value range of 0.5 to 0.9 bar. Likewise the upper limit is preferably 1.3 bar and in practice the equipment may be set to an upper value range of 0.9 to 1.5 bar. The system has two set points, high and low, in order to allow the use of the equipment at the surface and for descent as well as at depth. The low setting is used at the surface and during the descent whilst the high set point is utilised at depth and may be selected by means of a manual switch or may be automatically initiated by a pressure sensor. When the equipment is first turned on the low set point is automatically set at 0.7 bar and the high value at 1.3 bar. The purpose of providing a low set point for use at the surface and during descent is to avoid wasting oxygen, and also to allow the diver to conduct the descent at a reasonably fast rate without the oxygen pressure rising too high. The higher set point then gives oxygen-rich diving down to the normal maximum depths for sport diving (namely 45-50 meters) and also provides a reasonably oxygen-rich decompression, typically at about 81% O2 at 6 meters. The equipment is preferably provided with means for adjusting the high and low set points to allow divers to make adjustments to suit their particular purposes. Adjustments can be made under water if required.
In a preferred embodiment of the invention the sampling frequency at which the signals are processed is maintained at a level such that the valve controlled by the output signal from the sensors may be activated immediately in response to a change in the oxygen partial pressure.
The oxygen sensing arrangement preferably includes three oxygen sensors which are positioned spaced around a central location in a chamber, facing inwards so that when in the normal position of use, with the diver facing downwards, all three sensors face downwards, each being provided with an individual moisture deflector on the sensor face and waterproofing means on the connections and control circuits. It is also preferred that the sensors are positioned in such a way that the whole of the sensor is located in the chamber. In this way inaccuracies due to temperature gradients across the sensor face or body are eliminated. A vibration-proof locking device for the electrical connections, which will be described in more detail below is also provided.
The rebreather apparatus of the present invention preferably includes a source of a breathable diluent gas.
There may be provided means by which the diluent gas can be directed over the oxygen sensors upon introduction into the chamber, whereby to encourage drying of any moisture on the sensors.
The diluent gas may be air, Heliox or Trimix but is intended to be breathable at the target depth to afford a first open circuit emergency breathing gas source.
In another aspect the present invention provides self-contained breathing apparatus of the type comprising a mouthpiece, a counterlung, carbon dioxide extraction means, oxygen sensing and reintroduction means acting to maintain the oxygen partial pressure at or in the vicinity of a predetermined value to allow re-breathing of the gas from the counterlung, in which the counterlung is separated into two independent chambers one receiving exhaled gas from the mouthpiece and the other receiving gas from the carbon dioxide removing means, after introduction of oxygen, to act as a temporary store of gas reconditioned for breathing, in which the two counterlung chambers are connected to the mouthpiece by hose couplings incorporating two unidirectional valves orientated to ensure that air exhaled into the mouthpiece is directed only to the exhaled air counterlung and air inhaled through the mouthpiece arrives only from the reconditioned air counterlung, and in which the interconnection between an air hose and the counterlung is made by way of a T-coupling which is swivelable to allow free movement of the air hoses, which particularly facilitate assembly.
In any of the above aspects or embodiments there may be a T-coupling between each air hose and the associated counterlung which includes an internal baffle directing air within the hose to or from the counterlung and further acting as a moisture trap.
In the preferred embodiment of the invention the means for removing carbon dioxide from the exhaled gas encomprises a filter bed housed between circular permeable barriers in a cylindrical container having a central axial member.
The flexible container for receiving exhaled gas may be a shaped counterlung adapted to pass over the shoulder of a diver utilising the apparatus. The apparatus of the invention will in fact function satisfactorily whether the preliminarily shaped counterlung is mounted at the front or at the back of the diver, or passes over the shoulder. The pre-shaped xe2x80x9cover-the-shoulderxe2x80x9d configuration is preferred because this ensures that the counterlungs are as close as possible to the lung centroid in the majority of swimming positions thereby reducing static lung loading. The signal processing circuits preferably include monitors for detecting if the oxygen partial pressure departs from a predetermined range whereby to provide an alarm signal, and an audible alarm indicator is positioned close to the diver""s ear. This may be within the oxygen sensor casing or within the air hoses leading to and/or from the mouthpiece. These hoses may be afforded protection by a fabric sleeve which may be pre-shaped to a curved configuration. Such pre-shaped sleeves may be removable by the provision of elongate fasteners along their length.
To make it suitable for use by a diver the means for removing carbon dioxide, the oxygen sensors and control valve, a container housing a source of oxygen under pressure, and a container housing a breathable diluent gas are all supported on a panel having means by which the apparatus can be carried on the back of a wearer. This may be a buoyancy jacket, or a harness or straps. If straps or a padded harness are used then an additional buoyancy compensator preferably fits between the harness and the back frame of the apparatus of the invention. The counterlungs may be separate from the body harness, in which case the body harness passes through loops on the underside of the counterlungs. In other embodiments the counterlungs may be formed as the shoulder portion of the body harness. In such embodiments a heavier duty buckle is required at the bottom of the counterlungs. The panel may be part of a substantially rigid casing which protects the components from impacts or knocks in use and provides a more xe2x80x9cstreamlinexe2x80x9d external appearance.